Calibration Lab
Calibration Lab
The original planned use of the rangefinder system involved measuring only changes in distance (i.e. a "relative" system). In such use the "zero point" of each rangefinder, provided it is stable is of no interest. However, quite early in the project it was decided to use the rangefinders in a more ambitious way, that is to measure absolute range [1] .This means that the range from some reference point on the rangefinder to the target being measured has to be measured accurately.
The main result of this change of direction was the need for an accurately calibrated test range. Such a range has been constructed in the basement of the old 300 Foot Telescope control building. Several concrete monuments have been established by cutting through the concrete floor and pouring concrete cylindrical pillars well into the earth below. The distance between two of these pillars is the calibration range used to calibrate the rangefinder. The distance is approximately 19 meters and is measured exactly by building a track between two tooling balls, one on each of the two monuments. A laser interferometer is then used to count light fringes as a fixture holding a hollow retroreflector is moved along the track. The measurement is repeated at various times to check creep of the monuments and these measurements continue.
A layout of the calibration range is shown on Figure 1 and various photographs in Figure 2, Figure 3, Figure 4, and Figure 5.
[1] Absolute/Relative Measurement. J. M. Payne. 12/11/91. GBT Archives AS008.
This page was last modified on September 11, 1997.
